Conventional carbon fiber processing methods use small untwisted bundles of filaments, or “tows,” and low volumes of pre-stretched, fast-oxidizing polymer (with accelerants) or fibers that are composed with or incorporate an accelerant. The carbon fiber precursor materials for such processing methods are often specialty products intended specifically for carbon fiber production.
The automotive industry has not adopted widespread use of carbon fiber materials primarily because the cost of the carbon fiber material remains at relatively high specialty material prices, while widespread usage in automobile manufacturing would require relatively lower commodity pricing. While attaining such pricing, the material must meet the performance criteria required by the auto industry. The performance criteria prescribed by some automotive manufacturers for carbon fiber materials is that the material meet or exceed 400 ksi tensile strength and 40 Msi tensile modulus with at least 1% strain as minimum properties to encompass the automotive carbon fiber uses. In some semi-structural automotive composite applications carbon fibers with 250 ksi tensile strength and 25 Msi tensile modulus with at least 1% strain are sought.
Carbon fiber production begins with a carbonaceous precursor fiber material. A common carbonaceous precursor material is polyacrylonitrile (PAN). Specialty PAN precursor fibers are available with a variety of comonomers and accelerants. The comonomers are provided to impart desired properties to the precursor fiber and to the finished carbon fiber product. Commercial grade specialty acrylic fibers consist of a copolymer of acrylonitrile in combination with comonomers from various choices. The statistical copolymers usually contain 2-5 mol % comonomers. The comonomers are usually vinyl compounds with carboxylic acid (acrylic acid, methacrylic acid, itaconic acid) or their esters (methyl acrylate, methyl methacrylate) or their amides (acrylamide). These polymers are usually designed to have high molecular weight and narrow molecular weight distribution. These compositions are polymerized and solution spun into fiber form with significant draw down ratio (stretching), usually 14× or higher, achieved by steam stretching or other methods known in the art. Increased comonomer content helps to stretch and align the molecules along the fiber axis direction; however, that also increases the probability of chain scission during subsequent thermal processing of the carbon precursor fiber. Thus an optimally low comonomoner content is used. The fibers usually undergo thermal cyclization and oxidative crosslinking reaction at temperatures ranging from 180° C. to 300° C. These reactions are exothermic in nature and conventional art prefers to avoid overheating of the precursor fiber to control the chain scission reaction and melting of the fiber prior to rendering it to crosslinked intractable fiber. Overheating also causes thermal relaxation of the fiber and occasional ignition of the filaments. Thus keeping sufficient heat transfer in mind these specialty acrylic fibers are made of tow (bundle of filaments) of less than 80,000 filament counts.
Textile grade acrylic fibers are used in staple yarn form for clothing application. These fibers are also used in hand crafting (knitting and crochet), synthetic wool and flame resistant fabric applications. Because of its apparel usage, dying of the fiber is an important aspect. Thus chemical compositions mainly focus on comonomers that allow significant dye adsorption on the fiber surface. Vinyl acetate and methyl acrylate are commonly used comonomers with optional loading of vinyl chloride or vinylidene chloride for induction of flame retardant properties. Textile fibers are produced in large tow size (approx. 500,000 filament per tow or higher) and usually have lower molecular weight than the specialty acrylic carbon precursor fibers.
Textile PAN polymers are statistical copolymers of acrylonitrile polymerized in solvents such as dimethylformadide, dimethylsulfoxide, dimethylacetamide to produce a PAN solution that are processed directly to produce fiber without removal of the low-molecular weight oligomeric product. The presence of these low-molecular weight products in textile PAN fiber causes a broad molecular weight distribution in the commodity product, compared to the standard specialty acrylic PAN carbon precursor fibers (also known as specialty acrylic fibers or SAF). These textile fibers are not significantly stretched (3-5× draw-down ratio); rather at the end of a moderate degree of stretching the fibers are molecularly relaxed to obtain fiber with an unstrained amorphous phase where dye molecules can migrate to form colored textiles.
An important component of the carbon fiber production process is the oxidation/stabilization stage of the process. Accelerants are provided to accelerate the oxidation/stabilization process so as to reduce the time requirements for oxidation, which can be substantial and a time and production volume limiting factor of the carbon fiber production process.
The oxidation/stabilization process is complex and exothermic. In the case of PAN precursor fibers, upon heating the cyano side groups form cyclic rings with each other (cyclization reaction), and upon further heating in air these rings become aromatic pyridine. Oxygen molecules present in the air allows thermal dehydrogenation in cyclized rings to form the aromatic pyridine structures. Upon further heating adjacent chains join together to form ribbons, expelling hydrogen cyanide gas. Oxygen is also used to crosslink the ribbon structures through formation of ether linkages; oxidation is also known to form carbonyl and nitrone (nitrogen in cyclic structure bonds to atomic oxygen through dative bonding) structures. The stabilization process is highly exothermic and care must be taken to control or dissipate the generated heat.
During thermal oxidation the precursor polymer changes its structure in each oxidation zone due to cyclization and crosslinking reactions. The actual melt temperature of the polymer in fibers varies depending on the process conditions, and thermal history of the composition; however, in general the fusing temperature is higher after each pass in oxidation and the density of the fiber increases. To accomplish a higher rate of oxidation, temperatures in subsequent oxidation zones are gradually increased.
During the oxidation process the temperature of the fiber is required to maintain below its softening temperature to avoid inter-filament fusion. Sudden increases in the temperature of the filament lowers filament mechanical strength and often causes breakage of filaments that undergo mechanical stretch against extreme shrinkage force caused by cyclization and oxidative crosslinking reaction.
Stabilized PAN fibers with a high degree of oxygen uptake, to accomplish a high degree of crosslinking reactions, usually demonstrate increased fiber density. PAN precursor fibers have density of 1.18-1.20 g/cc; whereas oxidized PAN fibers can have densities in the range of 1.25-1.45 g/cc. Oxidized fibers with a high density range (>1.40 g/cc) exhibit significant flame retardancy.
After stabilization of the fibers, further heating in furnaces under inert (N2) atmosphere (a process called carbonization) expels nitrogen gas along with oxygen containing compounds, and other volatile organic tar forming compounds to form the carbon fibers with a higher degree of aromatic chemical structures.
The desire to increase production volumes has led to the widespread use of pre-stretched, specialty precursor fibers which include accelerants for accelerating the oxidation reaction. The presence of accelerant functionalities enhances the kinetics of thermal cyclization reaction of PAN. The precursor fibers are arranged into tows of about 100,000 deniers less and are passed rapidly through the oxidation oven usually maintained in a hot air atmosphere. Heating is applied and controlled to also enable the oxidation reaction to proceed. The application of such external heat results in an energy cost to the process. The stored heat in these tows (i.e. the heat that evolves during cyclization and oxidation reactions) require the fiber to be spread thinly to a fiber loading concentration of 100,000 deniers or less per inch of width in the stabilization ovens. This low fiber loading concentration requirement in oxidation, to avoid inter-filament fusion caused by heat evolved during precursor fiber oxidation, is at least partially responsible for the high cost of carbon fiber.